From 8b8537d30142a3d8b343cdd4e176a65ccdff1748 Mon Sep 17 00:00:00 2001 From: Stefano Brivio Date: Wed, 20 Sep 2023 16:39:11 +0200 Subject: [PATCH] dhcpv6: Properly separate domain names in search list To prepare the DHCPv6 domain search list option, we go over the flattened list of domains, and replace both dots and zero bytes with a counter of bytes in the next label, implementing the encoding specified by section 3.1 of RFC 1035. If there are multiple domains in the list, however, zero bytes serve as markers for the end of a domain name, and we'll replace them with the length of the first label of the next domain, plus one. This is wrong. We should only convert the dots before the labels. To distinguish between label separators and domain names separators, for simplicity, introduce a dot before the first label of every domain we copy to form the list. All dots are then replaced by label lengths, and separators (zero bytes) remain as they are. As we do this, we need to make sure we don't replace the trailing dot, if present: that's already a separator. Skip copying it, and just add separators as needed. Now that we don't copy those, though, we might end up with zero-length domains: skip them, as they're meaningless anyway. And as we might skip domains, we can't use the index 'i' to check if we're at the beginning of the option -- use 'srch' instead. This is very similar to how we prepare the list for NDP option 31, except that we don't need padding (RFC 8106, 5.2) here, and we should refactor this into common functions, but it probably makes sense to rework the NDP responder (https://bugs.passt.top/show_bug.cgi?id=21) first. Reported-by: Sebastian Mitterle Link: https://bugs.passt.top/show_bug.cgi?id=75 Signed-off-by: Stefano Brivio --- dhcpv6.c | 24 +++++++++++++++++------- 1 file changed, 17 insertions(+), 7 deletions(-) diff --git a/dhcpv6.c b/dhcpv6.c index fc42a84..58171bb 100644 --- a/dhcpv6.c +++ b/dhcpv6.c @@ -376,24 +376,34 @@ search: return offset; for (i = 0; *c->dns_search[i].n; i++) { - if (!i) { + size_t name_len = strlen(c->dns_search[i].n); + + /* We already append separators, don't duplicate if present */ + if (c->dns_search[i].n[name_len - 1] == '.') + name_len--; + + /* Skip root-only search domains */ + if (!name_len) + continue; + + if (!srch) { srch = (struct opt_dns_search *)(buf + offset); offset += sizeof(struct opt_hdr); srch->hdr.t = OPT_DNS_SEARCH; srch->hdr.l = 0; p = srch->list; - *p = 0; } - p = stpcpy(p + 1, c->dns_search[i].n); - *(p++) = 0; - srch->hdr.l += strlen(c->dns_search[i].n) + 2; - offset += strlen(c->dns_search[i].n) + 2; + *p = '.'; + p = stpncpy(p + 1, c->dns_search[i].n, name_len); + p++; + srch->hdr.l += name_len + 2; + offset += name_len + 2; } if (srch) { for (i = 0; i < srch->hdr.l; i++) { - if (srch->list[i] == '.' || !srch->list[i]) { + if (srch->list[i] == '.') { srch->list[i] = strcspn(srch->list + i + 1, "."); }