From 3dd6336ad0cb40e928745404ed72c41e4ac9c39e Mon Sep 17 00:00:00 2001 From: Joan Bruguera Date: Sun, 12 Feb 2023 20:06:08 +0000 Subject: [PATCH] resolved: Fall back to TCP if UDP is blocked If UDP is blocked on the system (e.g. by iptables or BPF), the kernel will return EPERM on some or all of the system calls (connect, sendmsg, etc.). In this case, try to fall back to TCP, which hopefully will not be blocked. --- src/resolve/resolved-dns-transaction.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/src/resolve/resolved-dns-transaction.c b/src/resolve/resolved-dns-transaction.c index 8d76630adb..3f994690e6 100644 --- a/src/resolve/resolved-dns-transaction.c +++ b/src/resolve/resolved-dns-transaction.c @@ -2079,7 +2079,9 @@ int dns_transaction_go(DnsTransaction *t) { log_debug("Sending query via TCP since it is too large."); else if (r == -EAGAIN) log_debug("Sending query via TCP since UDP isn't supported or DNS-over-TLS is selected."); - if (IN_SET(r, -EMSGSIZE, -EAGAIN)) + else if (r == -EPERM) + log_debug("Sending query via TCP since UDP is blocked."); + if (IN_SET(r, -EMSGSIZE, -EAGAIN, -EPERM)) r = dns_transaction_emit_tcp(t); } if (r == -ELOOP) {